Pre-mRNA processing is an essential step in eukaryotic gene expression. Constitutive splicing of intervening sequences (introns) from precursors of messenger RNAs (pre-mRNAs) is necessary to establish the correct reading frame for translation. Additionally, alternative inclusion of different coding sequences (exons) from the same transcript places splicing as a pivotal point of gene regulation. Mutations affecting both constitutive and alternative splicing are associated with a number of human diseases, including cancers. The goal of this proposal is to obtain and interpret structural information for the spliceosome, the very large macromolecular machine responsible for splicing catalysis. A three-dimensional (3D) structural understanding of this important molecule will be necessary to elucidate how this dynamic complex is able to precisely recognize very distant splice sites along a pre-mRNA and coordinate intron excision and exon ligation. Because the spliceosome is a dynamic complex composed of five structural RNAs (the U-rich small nuclear U1, U2, U4, U5 and U6 snRNAs) and on the order of 100 proteins, it presents challenges to structural studies. Cryo-electron microscopy (cryo-EM) provides a means to visualize this complicated machine. We will pursue a combination of EM labeling and biochemical characterization of purified splicesomes arrested between the two chemical steps of splicing chemistry to provide an interpretation of the cryo-EM structure. This will allow us to map spliceosome components on the structure to identify the pre-mRNA substrate and active site. These studies will move us closer to defining the mechanisms of splice site identification, spliceosome assembly, and splicing catalysis.